Snap acting thermostatic switches have long been used to protect motors, generators, transformers and like electrical components by breaking contact between the component and a power supply during an elevated transient temperature of the ambient and by re-establishing contact between the component and the power supply when the ambient temperature has cooled to a safe level. Contact is made and broken within the switch by a fixed contact and a movable contact connected to one end of a temperature responsive, snap action, bimetal blade. The blade is mounted in the switch so as to be cantilevered from its other end. There are of course many different switch designs in the prior art. For exemplary purposes, the fixed contact and the bimetal blade can be mounted on a pair of terminal strips that are mounted in a non-conducting case with the strips insulated from one another. As another example, the blade can be mounted on a basewall of an electrically conductive can and the fixed contact can be mounted on an electrically conductive lid that is insulated from the can. As still another example the switch can have an elongated terminal arm and a terminal lug mounted in the open end of a case in an insulated manner from one another. In such a switch the bimetal blade is cantilevered from the terminal lug and the fixed contact is connected to an end of the terminal arm that projects into the case.
The snap action of the bimetal blade is produced by a centrally located, cupped or dish-like portion which can be referred to as a snap acting depression. When the ambient temperature reaches an actuation temperature, a sudden reversal of the shape of the depression occurs to produce a deformed state of the bimetal blade. In the deformed state of the bimetal blade, the movable contact is spaced a distance from the fixed contact and the end of the blade, mounting the movable contact, is located against some point of contact or step on the switch. Depending upon the switch design, the step can be the terminal strip mounting the bimetal blade, a wall of the case adjacent to the terminal lug that in turn mounts the bimetal blade, or the basewall of the electrically conductive can mounting the bimetal blade. In any of the switch designs, the spacing of the contacts produces a circuit open condition of the switch in which contact between a power supply and an electrical component is broken. After a sufficient time has elapsed, and the ambient has cooled sufficiently to reach a reset temperature, the snap acting depression reverses to return the bimetal blade to an undeformed state to produce a circuit closed condition of the switch. In the circuit closed condition, the movable contact is located against the fixed contact and contact is re-established between the power supply and the electrical component.
For a variety of reasons, that are well known in the art, a particular bimetal blade design can only be specified as having a range of actuation temperatures and a range of reset temperatures. In order to insure the protection of the electrical component from elevated ambient temperatures, often, the actuation temperature is calibrated to an exact figure within the range of actuation temperatures. The calibration is performed on the switch in a heated temperature environment in which the ambient temperature is the desired actuation temperature. The blade is then prestressed by adjustment of a well known fulcrum-like calibration projection that is formed on the elongated member of the switch. The calibration projection bears against the snap acting depression, when the bimetal blade is in its undeformed state, with a sufficient force to cause the depression to suddenly reverse its shape or snap. Even though, when calibrated, such a switch can protect the component from elevated ambient temperatures, the operating temperature differential of the switch, between the actuation and the reset temperatures, can be far too broad relative to the safe operating of the component. This is because the reset temperature is not calibrated and as such, the reset temperature of the switch can be much lower than the safe operating temperature of the component. As can be appreciated, the disadvantage of this is that, although the ambient temperature can be at a safe level, the component remains idle until the ambient has cooled to the unnecesarily low reset temperature.
The present invention provides a method of forming a thermostatic switch with a calibrated operating temperature range in which both the actuation and reset temperatures are calibrated. The operating temperature range can therefore be selected to more realistically protect the electrical component, than prior art switches, by producing a calibrated operating temperature range that can be the desired, preferred operating temperature range of the electrical component. As will be discussed in greater detail hereinafter, this is accomplished by providing a blade that has an actuation temperature range that is above the desired actuation temperature and that has a reset temperature range below the desired reset temperature. The blade is then prestressed, as described above, to calibrate the actuation temperature. However, unlike the prior art, the calibrated actuation temperature is below the supplied range of actuation temperatures for the particular blade. After calibration of the actuation temperature, the blade is also prestressed, when in its deformed state, to upwardly calibrate the reset temperature, above the supplied range of reset temperatures, and towards the actuation temperature.
A switch formed in the manner described above can have a very narrow operating temperature differential to function with a component having a narrow operating temperature range requirement. It should be pointed out here that the prior art has provided thermostatic switches with small operating temperature differentials. These switches however, have shallow snap acting depressions of between about 0.0254 mm. and about 0.0408 mm. deep, and function more on a creep action of the contacts than a snap action at the limits of a calibrated operating temperature range. Thus, while such prior art switches have a small operating temperature differential, they do not have the exactly calibrated, narrow temperature range of a switch formed in accordance with the present invention.